Teacher efficacy is a topic of significance in mainstream education and various instruments have been developed to measure this construct. The available instruments however, are general both in terms of their subject matter and context. To compensate for this generality, the present study aims to develop a new teacher efficacy instrument whose items are specific to ELT classes. To this end, based on a thorough analysis of the existing literature and the researchers' conceptualization of typical ELT classes, a tentative theoretical model of ELT teacher efficacy was developed. This phase was followed by a series of classroom observations plus interviews with students, teachers, and experts. The model was then crosschecked against the results of the observations and interviews, and evolved into a scenario-based, Likert-scale ELT teacher efficacy instrument (ELTEI). The newly developed instrument was validated through administering it to 206 English language teachers, leading to some modifications in the model. The study hopefully can lead to a more meaningful interpretation of teacher efficacy in terms of context, language skills, students' age and proficiency, as well as teachers' perception in L2 settings.